The invention relates to a method of determining the monitor sensitivity of a radiation emitting arrangement, which monitor sensitivity relates a level of a minitor signal generated by the arrangement to a corresponding optical output level of the arrangement.
The invention more particularly, relates to a method of controlling a radiation emitting arrangement, the method comprising the steps of receiving a monitor signal from the arrangement and supplying a drive signal to the arrangement at such a level that a level of the monitor signal corresponds to a monitor reference level representing a desired optical output level of the arrangement.
The invention relates even more particular, to a system for controlling a radiation emitting arrangement, the system comprising means for receiving a monitor signal from the arrangement and drive means for supplying a drive signal to the arrangement at such a level that a level of the monitor signal corresponds to a monitor reference level representing a desired optical output level of the arrangement.
An important application of radiation emitting arrangements, and consequently for methods and systems as set forth above, is in optical recording and/or reproducing apparatuses such as the well-known Compact Disc (CD) players.
The invention still further relates to an optical recording and/or reproducing apparatus comprising a radiation emitting arrangement and a control system for supplying a drive signal to the arrangement in response to a monitor signal received from the arrangement. Those skilled in the art will readily appreciate that the invention is also of use in other applications, for example in optical communications.
A control system such as set forth above for use in an optical disc recording and reproducing apparatus is disclosed in European Patent Application EP-A2-0223576 to which U.S. Pat. No. 4,785,443 corresponds. In optical disc reproducing apparatus, the radiation source is typically a semiconductor laser which emits a powerful beam of infrared radiation when supplied with a drive current above a threshold value. Because the threshold value of the drive current is highly variable, notably with temperature, a photodiode is mounted with the laser so that the optical output power of the laser can be monitored directly and controlled by feedback.
Unfortunately, the efficiency of the optical coupling between the laser and the photodiode is highly variable, due in particular to tolerances in the manufacture of the laser/photodiode assemblies, so that an initial adjustment of all new control systems must be made at the factory. For the example of a CD player, this is typically done by inserting a reference disc of known reflectivity and adjusting the monitor signal gain to achieve a desired output from a photodiode array in the optical pick-up. In the known system this adjustment is made by means of a variable resistance in series with the monitor photodiode.
It is well known that such an adjustment process represents a significant addition to the manufacturing cost of a CD player or other apparatus, particularly since the adjustment in question is in many CD players the only manual adjustment not to have been automated or eliminated by improved design.
The problem is exacerbated by the fact that when using low-cost laser/photodiode assemblies the control system must cope with such a wide range of variation in monitor sensitivity that even a slight misadjustment, for example during after-sales service, can easily destroy the laser.